This invention relates generally to lead frames for use in integrated circuit packages. More particularly, it relates to a multi-layer lead frame which is well adapted for achieving fine lead pitch and/or multi-routing capability in integrated circuit packages.
As technology advances in the field of integrated circuits (IC's), high pin count complex IC's are moving toward smaller and smaller geometry's and are becoming more prevalent in semiconductor packages. These IC's are increasingly being packaged in multi-chip configurations that reduce the physical size package relative to single chip packages. Multi-chip packages require corresponding increases in the number of leads or fine pitch lead frames in order to facilitate electrical coupling the IC's to other circuit components on a circuit board, for example.
There have been a variety of package configurations employed in the past that are capable of handling IC's with a high number of input/output (I/O) terminals. By way of example, Tape Automated Bonding (TAB) package structure includes many very fine wiring traces surrounding and bonded to the IC. These fine traces are covered with protective tape and are subsequently connected to larger traces or a lead frame for connection to other components. The advantage of a TAB package is that IC's with a relatively large number of I/O terminals can be coupled with the fine geometry's obtained with the traces at a relatively low cost. Despite the advantages of TAB, there are some noteworthy disadvantages such as not being as structurally stable as most plastic encapsulated packages, and they typically are formed on a substrate or circuit board such that they are not shipped as individual units like encapsulated packages.
The more popular plastic packages, such as Quad Flat Packages (QFP) and Ball Grid Arrays (BGA) typically use lead frames to electrically couple the IC. Standard lead frames used with QFP's are typically about 6 mils thick and therefore it is difficult to obtain very fine lead pitch at this thickness. A general manufacturing rule of thumb is that the distance between the leads can be as close as the thickness of the leads. Since most lead frames are made by stamping or etching, attempts at reducing the lead pitch much further requires very fine stamping dies which, at these small dimensions, are prone to break or wear out quickly. Etching also has a size limitation since it is often difficult to obtain uniformly smooth sidewalls in deep narrow channels. One approach for overcoming these problems that works well is described in my co-pending application Ser. No. 08/287,872 entitled "A Fine Pitch Lead Frame and Method for Manufacturing Same" filed Aug. 9, 1994 (now U.S. Pat. No. 5,454,905) which is incorporated herein by reference. The present application seeks to provide a lead frame structure that is capable of provided a very fine lead pitch arrangement and having multi-layer routing capability such that the leads have better access all sides of IC's mounted in multi-chip packages.